Experimental and theoretical studies on the (co)cyclotrimerization of alkynes (and ethylene) in a TpRh compound

Abstract

The reactions between the Rh(I) ethylene complex TpRh(C₂H₄)₂ (1; Tp = hydrotris(pyrazolyl)borate), and the alkynes di-tert-butylacetylene dicarboxylate (DTBAD), acetylene, phenylacetylene, and methyl propiolate (MP) have been studied, and the results compared with those obtained previously for dimethylacetylene dicarboxylate (DMAD). Electron withdrawing groups (DTBAD) at both termini of the triple bond stabilize η⁴-diene-Rh(I) species, while terminal alkynes (acetylene, phenylacetylene) easily lead to η³-allyl-Rh(III) species. The terminal alkyne methyl propiolate, with only one electron withdrawing substituent, exhibits an intermediate behavior, forming both η⁴-diene-Rh(I) and η³-allyl-Rh(III) species. Two types of intermediate octahedral TpRh(III) rhodacycles have been detected and characterized by NMR. Mechanistic investigations by DFT are also in agreement with the intermediate role of these types of species, which form via oxidative coupling of compounds of type [TpRh(C₂H₄)(C₂R₂)] and [TpRh(C₂R₂)₂]. Ligand exchange reactions to form these species from 1 are instrumental in the outcome of the reactions.

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Introduction

The metal-mediated [2 + 2 + 2] cyclotrimerization of alkynes is a straightforward procedure for the synthesis of highly substituted benzene derivatives,¹ with interesting applications in organic synthesis.² In the presence of an olefin, a cyclohexadiene derivative may be formed instead, by the coupling of two molecules of alkyne and one of the alkene.³ Several mechanisms have been proposed for these reactions, with the common steps of the initial formation of a metallacyclopentadiene⁴ by the coupling of two molecules of alkyne in the metal complex, and the evolution of the former, by different proposed routes,⁵ to the final compounds (Scheme 1).

Scheme 1 Schematic, simplified representation of the evolution of a metallacyclopentadiene in the presence of (a) ethylene or (b) acetylene.

In our efforts to generate substituted benzenes via alkyne cyclotrimerization, we previously reported the reactions of two Tp^Me₂Ir(I) complexes (Tp^Me₂ = hydrotris(3,5-dimethylpyrazolyl)borate) with dimethyl acetylenedicarboxylate (DMAD).^6a,b In none of these cases was the cyclotrimerization product observed, which we attributed to the reluctance of Tp^Me₂Ir(III) species to undergo reductive elimination. In contrast, in previous studies involving the reactivity of complex TpRh(C₂H₄)₂ (1) (Tp = hydrotris(pyrazolyl)borate) toward DMAD,^6c we observed the formation, albeit in low yield, of hexakis(methoxycarbonyl)benzene (C₆(CO₂Me)₆), the product of [2 + 2 + 2] cyclotrimerization of DMAD. This difference is likely due to the greater propensity of TpRh(III) intermediates to undergo reductive elimination to Rh(I) species. In fact, the reaction of complex 1 with three equivalents of DMAD resulted in a mixture of rhodium complexes along with the formation of the substituted benzene 2e (Scheme 2). Complexes 2a, 2b, and 2c are Rh(I) species featuring η⁴-diene ligands, either open-chain (2a) or cyclic (2b and 2c). In contrast, complex 2d is an allyl Rh(III) species.

Scheme 2 Reaction of TpRh(C₂H₄)₂ (1) with 3 equivalents of DMAD (C₆H₆, 25 °C, 4 h) (isolated yields in parentheses).

We have now extended this study to a broader range of alkynes, including di-tert-butylacetylene dicarboxylate (DTBAD), acetylene, phenylacetylene, and methyl propiolate (MP). In addition, we have carried out a detailed mechanistic investigation of the reactions of complex 1 with acetylene and DMAD using density functional theory (DFT) calculations.

Results and discussion

Reaction of TpRh(C₂H₄)₂ (1) with di-tert-butylacetylene dicarboxylate (DTBAD)

To evaluate the influence of steric effects exerted by substituents at the alkyne triple bond without altering their electronic properties, the reaction of complex 1 with 3 equivalents of di-tert-butylacetylene dicarboxylate (DTBAD) was carried out in benzene at room temperature. The results are summarized in Scheme 3. The major product, the Rh(I) species 3, is analogous to compound 2a observed in the DMAD system.^6c Two minor species, 4 and 5, also share the same cyclohexadiene core as 2b, formally resulting from the coupling of two alkyne units and one ethylene fragment. However, the substituents on the organic ring occupy different positions relative to the double bonds, indicating distinct regioisomeric outcomes. These differences are attributed to steric effects imposed by the bulky tert-butyl groups. Surprisingly, the cyclotrimerization product was not detected in the reaction mixture. Purification of the three compounds was achieved by column chromatography (although 4 was isolated in admixture with small amounts of 5). The excess DTBAD was also recovered through the same method.

Scheme 3 Reaction of TpRh(C₂H₄)₂ (1) with DTBAD (in this and subsequent Schemes, isolated yields are shown in parentheses unless otherwise stated).

Compounds 3, 4, and 5 yielded single crystals upon slow evaporation of the eluent mixture (hexane/diethyl ether) and were characterized by X-ray crystallography (Fig. 1 and S2 in the SI). Their solid-state molecular structures reveal several close contacts between the tert-butyl groups and the pyrazolyl rings of the Tp ligand, highlighting notable intramolecular steric interactions. These close contacts are forced by the steric crowding around the Rh metal center and might also play a role for the different reaction mixture compositions obtained compared to the reactions with DMAD. For instance, in compound 3 there is a CH⋯π interaction⁷ between C(24) of a ^tBu group and the pyrazolyl ring N5–N6–C7–C8–C9 of the Tp ligand. This interaction is characterized by a H⋯centroid distance of 2.95(4) Å and a C–H⋯centroid angle of 158.7(8)°. Similar parameters can be measured in the crystal structures of 4 and 5. Other geometrical parameters, i.e. bond lengths and bond angles, are comparable to the related species discussed previously for the DMAD derivatives.^6c

Fig. 1 ORTEP diagrams of compounds 3 (up) and 5 (down). 50% thermal ellipsoids are shown. Most hydrogen atoms have been omitted for clarity. An ORTEP diagram of compound 4 is shown in Fig. S2.

To gain insight into the mechanism of formation of the different rhodium species, we monitored the reaction of complex 1 with 2.5 equivalents of DTBAD in C₆D₆ at 8 °C by ¹H NMR spectroscopy (Scheme 4). A new major species was observed, corresponding to the intermediate metallacyclopentene 6,^5,8 as indicated by its characteristic NMR pattern: a resonance centered at 3.97 ppm (AA′BB′ spin system, ethylene) and multiplets at 3.07, 3.02, 2.72, and 1.92 ppm, corresponding to the four inequivalent protons of the Rh-CH₂CH₂– moiety. Additionally, a second intermediate, the rhodacyclopentadiene 7, was identified by a singlet at 4.32 ppm, attributed to the four equivalent protons of the freely rotating ethylene ligand.

Scheme 4 Detection of intermediate species 6 and 7 by ¹H NMR and their evolution to 3 and 8, respectively.

Under these conditions, intermediates 6 and 7 were present in a 3 : 1 ratio, based on the integration of their respective ethylene ligand signals in the ¹H NMR spectrum. After 30 minutes at 8 °C, intermediate 7 disappeared and compound 8 was detected, while intermediate 6 underwent intramolecular transformation into the final open η⁴-diene product 3, reaching full conversion after 4 hours at room temperature. The final ratio between 3 and 8 reflects the initial 3 : 1 ratio between 6 and 7, which are intermediates analogous to those previously described in the DMAD system.^6c

On the other hand, the mechanistic origin of compounds 4 and 5 remains uncertain due to the rearrangement observed in their carbocyclic skeletons. However, interconversion between the two isomers after formation can be ruled out, as no changes were detected upon standing the pure compounds in C₆D₆ at room temperature or upon heating at 60 °C for 24 hours. Similarly, heating compound 8 at 80 °C for 24 hours did not lead to its conversion into either of the two isomers.^9a Nevertheless, the isomerization^9b of this framework at a specific stage of the formation process may be favoured by the release of steric strain around the metal center, following a progression from 8 to 5 to 4 (Fig. 2).

Fig. 2 Schematic representation of the Rh-diene moieties of compounds 8, 5 and 4. [Rh] = TpRh; R = CO₂^tBu.

The molecular structure of compound 8 was postulated based on NMR analysis and subsequently confirmed by X-ray crystallography (see SI and Fig. S4 for details). We propose that concentration of the alkyne plays a significant role in the formation of the intermediate metallacycles 6 and 7, as well as in determining their relative ratio. Evidence for this effect was obtained by mixing complex 1 and 3 equivalents of DTBAD as solids in an NMR tube and heating the mixture at 50 °C for 1 hour. During this time, the alkyne acted as the solvent due to its low melting point (33–34 °C). After cooling, CDCl₃ was added to the tube, and the ¹H NMR spectrum revealed the formation of compounds 3 and 8 in an approximately 1 : 1 ratio. These results suggest that steric factors may hinder the exclusive formation of 8. Notably, even in the case of the less bulky DMAD, the rhodacyclopentene intermediate of type 6 is consistently formed, regardless of the amount of alkyne used (1–8 equivalents). This observation indicates that coupling between ethylene and the alkyne is highly favourable, even in the presence of excess alkyne.

The reaction between complex 1 and DTBAD was also carried out in dichloromethane and THF. Notably, when THF was used under the same conditions described in Scheme 3 (3 equivalents of DTBAD, room temperature, 4 h), compound 8 was detected as the sole product in the crude mixture (spectrocopic yield (≥90%) and 24% isolated yield). This change in reactivity implies a role of THF medium, likely involving the replacement of the ethylene ligand in metallacycle 6, which leads to 8 by reaction with further alkyne, as already reported in similar systems.¹⁰

Isolation of a stable analogue of 6 ¹¹ with water acting as a stabilizing agent^6a,b is achieved when the reaction of complex 1 with 1 equivalent of DTBAD is carried out in a THF/H₂O mixture (Scheme 5). The resulting compound 9 was structurally characterized by X-ray crystallography (Fig. 3). The molecular structure of 9 reveals an almost ideal octahedral geometry around the rhodium center, with minimal distortion and bond angles close to 90°. The C–C bond distances of C(12)–C(18) (1.351(2) Å) and C(10)–C(11) (1.536(2) Å) are consistent with double and single bond character, respectively. In the solid state, compound 9 forms a dimer through a characteristic hydrogen bonding motif of moderate strength, O⋯H–O–H⋯O (see Fig. S6 in the SI for details).¹²

Scheme 5 Synthesis of the rhodacyclopentene 9.

Fig. 3 ORTEP diagram of compound 9. 50% thermal ellipsoids are shown. Most hydrogen atoms have been omitted for clarity.

We have also conducted experiments aimed at obtaining the [2 + 2 + 2] cyclotrimerization product of the DTBAD monomer. In the case of DMAD, the corresponding benzene derivative is formed upon reaction of compound 2a with an excess of alkyne, following displacement of the open-chain diene and subsequent coupling of three DMAD molecules within the TpRh coordination sphere.^6c In contrast, heating a benzene solution of compound 3 with 6 equivalents of DTBAD at 65 °C for 12 hours resulted in no observable reaction. However, under identical conditions using 6 equivalents of DMAD, the formation of the previously reported compounds 2c and 2e was observed, along with dissociation of the organic diene 10 as well as compound 11, which arises from the coupling of diene 10 with two DMAD molecules (Scheme 6). This type of transformation has already been described in the DMAD system.^6c

Scheme 6 Reaction of 3 with excess of DMAD (isolated yields are shown for rhodium species).

The facile generation of compound 2c in this reaction suggests that, while 3 reacts with DMAD, DTBAD is too bulky to displace the coordinated diene and subsequently incorporate three DTBAD molecules. This steric hindrance likely prevents the formation of benzene derivatives analogous to 2c and 2e. Nevertheless, the detection of rhodacyclopentadiene 7 at low temperature (Scheme 4), bearing four tert-butyl substituents, demonstrates that such crowded intermediates are feasible. Furthermore, the [2 + 2 + 2] cyclotrimerization product of the DTBAD monomer, hexa-tert-butylbenzene hexacarboxylate, is a known compound and can be prepared catalytically using other metal systems.¹³

The formation of compound 11 is proposed to arise from the activation and coupling of the diene ligand in compound 3 with two additional alkyne molecules. This transformation follows a mechanism analogous to that previously postulated for the DMAD system.^6c It involves the generation of a rhodacyclopentadiene intermediate via oxidative coupling of two DMAD molecules, along with the η²-coordinated diene through its unsubstituted olefin moiety. Subsequent insertion of this olefin into the Rh-C bond leads to a rhodium–cycloheptadiene intermediate, which ultimately rearranges to form the (η³-allyl)(η¹-allyl)Rh(III) complex 11.

Reaction of TpRh(C₂H₄)₂ (1) with acetylene

Treatment of 1 with acetylene (bubbling for a minute) in benzene or dichloromethane gave almost quantitative amounts of the Rh(III) compound 12 (Scheme 7). It is noteworthy that the analogous compound was only a minor species in the reaction between 1 and DMAD (2d in Scheme 2), and when alkyne DTBAD was employed, no allylic compound was observed (Scheme 3). Moreover, no formation of cyclic organic products such as benzene or cyclohexadiene was observed by monitoring the reaction with acetylene in CD₂Cl₂ in an NMR tube.

Scheme 7 Reaction of 1 with acetylene.

Furthermore, no rhodacyclopentene or rhodacyclopentadiene intermediates were detected when the reaction was carried out in C₆D₆. However, performing the reaction in acetonitrile allowed the isolation of the rhodacyclopentene 13, as an MeCN adduct, in good yield (Scheme 8). In contrast, when a THF/H₂O mixture was used, compound 12 was observed as the major product, with no trace of the aquo-adduct. This outcome suggests a high thermodynamic stability of the allyl product 12 and its facile formation in the absence of an appropriate stabilizing agent.

Scheme 8 Formation of the acetonitrile adduct of a rhodacyclopentene intermediate.

The reaction of complex 1 with deuterated acetylene in benzene was also carried out with the aim of confirming the structure of the product and gaining mechanistic insights into the transformation (Scheme 9).

Scheme 9 Preparation of 12-d₄ by reaction of 1 with acetylene-d₂ and the pathway proposed for the transformation.

As expected, the ¹H NMR spectrum of compound 12-d₄ showed, aside from the resonances corresponding to the Tp ligand protons, only four signals in the 1.50–5.50 ppm region. These signals account for the CH₂–CH allylic fragment and the Rh-CDH proton, confirming the proposed structure. Scheme 9 illustrates the pathway proposed for the formation of compound 12, based on mechanisms previously described for related Rh and Ir complexes.^6a,c,8a,14

Compound 12 represents a bis(allyl) complex, analogous to 11, but with identical composition at both ends of the carbon chain (η¹-allyl and η³-allyl). The NMR data for 12-d₄ indicates that no exchange between the two allyl termini occurs on the NMR timescale, even upon heating the sample to 80 °C for 2 hours.

We also investigated the reaction of complex 1 with acetylene in the presence of excess ethylene. The outcome of the reaction was found to depend on the relative concentrations of the two gases. Optimal results were obtained when ethylene was bubbled through the solution of compound 1 for 2.5 minutes at −60 °C, followed by a brief bubbling of acetylene for another 2.5 minutes, without interrupting the ethylene flow. Under these conditions, a new compound, 14, was obtained as the major product (>90%) along with a variable amount of compound 12 (Scheme 10). As the amount of acetylene increased, compound 12 became the predominant species, and in some cases, the only product detected in the reaction mixture.

Scheme 10 Reaction of 1 with acetylene in the presence of ethylene. Spectroscopic yield of 14 shown in parentheses.

Compound 14 was isolated by column chromatography using pentane as the eluent; however, it could not be obtained in a completely pure form. Nevertheless, it was fully characterized by spectroscopic and analytical methods.

In the ¹³C{¹H} NMR spectrum, the Rh-CH₂ moieties appear as a doublet at δ −17.4 ppm (¹J_CRh = 16 Hz). Coordination of the double bond to the rhodium center is evidenced by the chemical shifts of the corresponding CH units: 3.94 ppm in the ¹H NMR and 69.7 ppm in the ¹³C{¹H} NMR spectrum, values consistent with coordinated olefins.

An iridacyclohexene analogous to compound 14, bearing CO₂^tBu substituents on the olefin moiety, was previously proposed as intermediate in the formation of the iridium version of diene 3 (Scheme 4). To investigate whether a similar diene product could be obtained in this case, a solution of compound 14 in C₆D₆ was heated at 60 °C and monitored by NMR. After 3 hours, complex 14 was completely transformed into a new compound, 15 (Scheme 11). Prolonged heating at 60 °C for 24 hours did not result in further changes. Compound 14 also evolved into 15 at 25 °C over 72 hours, although the reaction under these milder conditions was not clean.

Scheme 11 Formation reaction of 15 from 13 and 14.

Indeed, the formation of compound 15 is analogous to that of 3, and similarly involves a β-H elimination followed by hydride migration to the Rh-CH₂ terminus of the organic chain (Scheme 4). In the ¹H NMR spectrum of 15, the –CH=CH₂ fragment coordinated to rhodium appears at δ 5.02 ppm (q, ³J_HH ∼ 7 Hz, CH), 2.01 ppm (dd, ²J_HH ∼ 3.2 Hz, ³J_HH ∼ 7 Hz, CH=CHH), and 1.39 ppm (m, CH=CHH). In the ¹³C{¹H} NMR spectrum, the signals corresponding to this fragment are observed as doublets at 89.5 ppm (–CH) and 29.6 ppm (CH₂), due to coupling with rhodium. Additionally, the ¹H NMR spectrum shows that the CH₂ protons of the ethyl group are diastereotopic, appearing as two distinct multiplets at δ 1.45 and 1.00 ppm.

Finally, to determine whether compound 13 evolves into 14 in the presence of ethylene, the reaction was carried out in CH₂Cl₂ by bubbling ethylene for 3 minutes at −20 °C, followed by heating at 60 °C. After 5 hours, complete transformation of 13 into compound 15 was observed. Notably, when the reaction was performed at 25 °C, compound 15 was also formed, and 14 was not detected under either condition (Scheme 11).

Reaction of TpRh(C₂H₄)₂ (1) with phenylacetylene

Additional evidence for the distinct reactivity of terminal versus internal alkynes is provided by the reaction of compound 1 with phenylacetylene, which predominantly yielded compound 16, as depicted in Scheme 12. Minor by-products were detected in the crude mixture by ¹H NMR spectroscopy; however, all attempts to isolate and characterize them were unsuccessful.

Scheme 12 Reaction of 1 with phenylacetylene.

Reaction of TpRh(C₂H₄)₂ (1) with methylpropiolate (MP)

Methyl propiolate (MP) exhibited an intermediate reactivity between acetylene and DMAD, predominantly affording the allyl rhodium complex 19, along with minor amounts of compounds 17 and 18 (Scheme 13), in a manner analogous to the reaction with DMAD.

Scheme 13 Reaction of 1 with methyl propiolate.

Interestingly, the formation of compounds 16 and 19 involves the regioselective activation of phenylacetylene and MP, respectively, in such a way that the Ph and CO₂Me substituents are positioned at the β-position relative to the rhodium center. This contrasts with previously reported Tp^Me₂Ir complexes derived from MP, where alkyne insertion occurs regioselectively to place the CO₂Me group at the α-position with respect to the iridium center.^6a,15

Computational study

Further insight into the reactivity of 1 with alkynes and ethylene was sought by Density Functional Theory methods (SMD(dichloromethane)-ωB97X-D/SDD(Rh)/cc-pVTZ//ωB97X-D /SDD(Rh)/6-31G(d,p)). This study focuses on the different outcome of reactions of 1 with acetylene and dimethylacetylenedicaboxylate (DMAD), as two relevant examples of the various alkynes used in this and previous^6c experimental work. In the first case, the allyl Rh(III) complex 12, is the main reaction product, whereas in the second case the electron poor alkyne DMAD promotes formation of Rh(I) complexes relevant to (co)cyclotrimerization reactions.

The mechanistic proposals discussed here are based on the experimental observations described above and elsewhere,^6c as well as on previous mechanistic proposals on related metal systems.^5,16 Oxidative coupling in either bis-alkyne or mixed alkyne–ethylene complexes to form rhoda-cyclopentadienes or -cyclopentenes respectively is an accepted initial step in the reactivity of these systems and, as we will show in the following paragraphs, the outcome of the reactions of 1 with alkynes and ethylene will depend on the relative amount of species of type [TpRh(C₂H₄)(C₂R₂)] and [TpRh(C₂R₂)₂] present in the reaction mixture that can undergo oxidative coupling. Therefore, we begin by describing the formation of the former species from the bis-ethylene complex 1 (Fig. 4).

Fig. 4 Stabilities of the mixed alkyne–ethylene and bis-alkyne complexes A and B (R = H, solid blue trace) and A′ and B′ (R = CO₂Me, dotted red trace, data in parenthesis) relative to 1 (ΔG_qh in dichloromethane, kcal mol⁻¹). *Bond dissociation energies (ΔH°) of ethylene in A and A′ are used to approximate ligand elimination in dissociative exchange.

Ligand exchange

Substitution of one or two ethylene ligands of 1 by acetylene to afford the mixed acetylene–ethylene species [TpRh(C₂H₂)(C₂H₄)] (A) and the bis(alkyne) complex B, respectively is almost thermoneutral (ΔG_qh in dichloromethane = −0.8 and 1.1 kcal mol⁻¹ from 1; see Fig. 4 and the computational details). Ligand substitution can occur by either associative or dissociative pathways. In an associative mechanism coordination of acetylene, first to 1 and in a second stage to A, requires changing the coordination mode of their Tp ligands from κ³ to κ², which is facile in both cases, with low energy barriers (ΔG^‡_qh) of 2.2 and 2.0 kcal mol⁻¹ from the corresponding κ³ complexes, respectively. However, inspection by relaxed Potential Energy Surface scans of the dissociation of ethylene and acetylene from intermediates of type [κ²-TpRh(C₂H₄)_2−n(C₂R₂)_1+n] (n = 0, 1), containing three unsaturated ligands, indicates that the associated energy barriers are higher than those in the dissociative pathway. Thus, acetylene coordination to κ²-A yields an 18-electron intermediate with two acetylenes and one ethylene, which lies 17.4 kcal mol⁻¹ above the origin (ΔG_qh). But, while the reverse reaction, acetylene dissociation, is almost barrier-less, dissociation of ethylene on the way to B adds more than 20 kcal mol⁻¹ to the overall barrier of this route, which amounts to more than 30 kcal mol⁻¹ from A (Fig. S7 and Fig. S7a–e in the SI).

On the other hand, energy barriers for the dissociative pathway, which have been estimated as the bond dissociation energies of ethylene from the 16-electron fragments [TpRh(C₂H₄)] and [TpRh(C₂H₂)], are 22.7 and 21.3 kcal mol⁻¹, respectively.

Substitution of ethylene by DMAD in 1 affords A′, which is 9.6 kcal mol⁻¹ below the origin. It follows from this result that DMAD is a better ligand for the TpRh^I moiety than acetylene. Replacement of a second ethylene by DMAD offers no extra stabilization, but B′ remains more stable than 1 by 6.7 kcal mol⁻¹. We have not been able to locate transition states associated with ligand exchange in an associative mechanism but, analogously to the acetylene case, elimination of ethylene from the corresponding species [κ²-TpRh(C₂H₄)₂(C₂R₂)], along the step sequence from A′ to B′ makes the overall energy barrier for this route higher than that found for the dissociative pathway, in which dissociation of ethylene from A′ requires 19.6 kcal mol⁻¹. Note that dissociation from A′ is easier than from A, which is in line with the preference of the TpRh(I) fragment for DMAD.

Oxidative couplings. Formation of metallacycles

Oxidative coupling at A to give the rhodacyclopentene C has a barrier of 17.9 kcal mol⁻¹ and it is exergonic by 21.2 kcal mol⁻¹ (Fig. 5). Transformation of B into the rhodacyclopentadiene D is both faster, ΔG^‡_qh = 10.5 kcal mol⁻¹, (overall free energy variation from 1), and more exergonic, ΔG_qh = −37.0 kcal mol⁻¹.¹⁷ The analogous DMAD-containing species A′ and B′ evolve to metallacycles C′ and D′ through similar energy barriers than their acetylene counterparts, with oxidative coupling at the bis(alkyne) species being again faster and more favorable thermodynamically than at the mixed alkyne–ethylene complex (ΔG^‡_qh = 6.6 vs. 16.3 kcal mol⁻¹ and ΔG_qh = −58.0 vs. −35.3 kcal mol⁻¹, respectively). These results see precedent in related systems.^5g In our case, oxidative coupling at the bis(alkyne) complexes B and B′ is faster than regeneration of species A but, more importantly, oxidative coupling at both A and B is not reversible, with the resulting metallacycles being further stabilized by barrier-less coordination of ethylene or alkyne. While both ligands compete to form the corresponding adducts, coordination of ethylene is more favorable thermodynamically.

Fig. 5 Gibbs energy profile in dichloromethane (kcal mol⁻¹) for the formation of rhodacycles C and D via oxidative coupling in the acetylene system (R = H). Data for A′–D′, with DMAD (R = CO₂(Me)) is given in parentheses. DFT-optimized geometries of the corresponding transition states for the acetylene system are also shown (hydrogen atoms on the Tp ligands are omitted for clarity).

Alternative pathways to the formation of the alkyne and ethylene adducts of species C and D imply oxidative coupling at saturated complexes of type [κ²-TpRh(C₂H₄)_2−n(C₂R₂)_1+n], i.e. the aforementioned intermediates in associative ligand exchange, followed by regain of the κ³-Tp coordination. A related mechanism has been hypothesized to explain the indenyl effect and the influence of the alkyne observed in the cyclotrimerization of acetylene catalysed by CpRh and Cp′Rh (Cp′ = indenyl) complexes.^16a However, in our case the barriers for oxidative coupling at these saturated intermediates exceed those found from complexes A and B (see Fig. S8 in the SI).

(Co)cyclotrimerization

The following step to account for the reactivity of 1 with alkynes is C–C coupling of η²-coordinated ethylene or alkyne to the metallacycles of C and D. Acetylene insertion into the Rh-C_sp2 σ-bond of C·C₂H₂ to yield E (Fig. 6) is facile, having a barrier of 11.3 kcal mol⁻¹, and very exergonic (ΔG_qh = −30.9 kcal mol⁻¹). E can also be formed by ethylene insertion into one Rh-C_sp2 σ-bond of D·C₂H₄, which has a barrier of 10.5 kcal mol⁻¹, comparable to the above.¹⁸

Fig. 6 Gibbs energy profile in dichloromethane (kcal mol⁻¹) for the formation of 12, including the structural representation of some relevant computed intermediates and transition states and the DFT-optimized geometry of E (with hydrogen atoms on the Tp ligands omitted for clarity). Data in parentheses are for the analogous process with the DMAD system.

Thermodynamic stability of E arises from coordination of the C_γ=C_δ double bond of the metallacycle with the metal to complete its 18-electron count. E yields 12 via rhodium hydride F, through a steps sequence that starts by isomerization of E through a low barrier to E_ag, which features a β-C–H⋯Rh agostic interaction. β-Elimination to afford the hydride complex F is almost barrier-less and takes place without change in the oxidation state of the metal. Further migration of the hydride to the Rh-C_sp2 carbon has a barrier of 19.5 kcal mol⁻¹. Interestingly, this step does not produce the reductive elimination product, hexatriene observed in cyclocopolymerization of ethylene and acetylene with CpCo catalysts.^5g,19 Instead, a new intermediate, G, forms, for which we propose a Rh(III) metallacyclopropane formulation based on a localized orbital analysis of its electronic structure (see Fig. S9 in the SI).²⁰ Finally, G relaxes easily to the allyl-Rh(III) product 12, observed experimentally. An alternative, less favorable pathway to 12 involves hydrogen migration from coordinated ethylene to one Rh-C_sp2 carbon of the rhodacycle of D, followed by C–C coupling at an intermediate, H, featuring alkenyl and 1-butadienyl ligands.^5g The barrier for hydrogen transfer is 13.8 kcal mol⁻¹ from D·C₂H₄, compared to the 10.5 kcal mol⁻¹ found for ethylene insertion. A reason for this difference may be the Rh(V) character of the transition state for the former transformation (see Fig. S10 the SI).²¹ When the DMAD system is considered E′ can be accessed either via insertion of DMAD in C′·DMAD, with an energy barrier of 8.8 kcal mol⁻¹, or via ethylene insertion in D′·C₂H₄ through a barrier of 12.1 kcal mol⁻¹. Contrary to the acetylene system, the latter step is almost thermoneutral (ΔG_qh = 0.8 kcal mol⁻¹). Regardless, the species analogous to 12 in the DMAD system, which was formed in reactions of metallacyclopentenes of type C·L (L = H₂O, MeCN) and DMAD,^6c is accessible from E′, according to the calculations, through a mechanism parallel to that described with acetylene, albeit the overall barrier of 18.1 kcal mol⁻¹ (from E′) renders this transformation less favorable kinetically than in the acetylene system (ΔG^‡_qh = 10.6 kcal mol⁻¹).

To account for the competing cyclotrimerization reaction, evolution of D·C₂H₂ was studied. Although no cyclotrimerization products have been detected with acetylene, [4 + 2] cycloaddition is predicted to occur at D·C₂H₂ through a synchronous transition state that collapses directly into the Rh(I) species [TpRh(η⁴-C₆H₆)] (see Scheme 2 and Fig. 7).^5c,f,16b This transformation is not reversible (ΔG_qh = −64.9 kcal mol⁻¹) and the energy barrier from D·C₂H₂ is a mere 3.8 kcal mol⁻¹, which is significantly lower than that found for insertion of ethylene in the metallacycle of D·C₂H₄.

Fig. 7 Gibbs energy profiles in dichloromethane (kcal mol⁻¹) for the final steps in the cyclotrimerization of acetylene and DMAD (dashed trace-data in parentheses-) from D·C₂R₂ (R = H, CO₂Me) and DFT optimized geometries of the transition states for the coupling of the third alkyne (hydrogen atoms of the Tp ligands and CO₂Me groups -transparent- omitted for clarity). Energies are relative to 1 + 3C₂R₂–2C₂H₄.

Cyclotrimerization of DMAD begins, at variance with the acetylene system, with insertion of the coordinated molecule of alkyne into one of the Rh-C_sp2 σ-bonds of D′·DMAD to yield the metallacycloheptatriene J′ (Fig. 7), followed by reductive coupling to afford the corresponding Rh(I) compound TpRh(η⁴-C₆R₆) (R = CO₂Me). J′ is related to intermediates proposed in the Schore mechanism for the cyclotrimerization of alkynes,^1j but with the electron count of the metal being completed by η²-coordination to one C=C double bond of the metallacycle, similarly to E and E′. The overall transformation is exergonic by 57.0 kcal mol⁻¹ from D′·DMAD, while DMAD insertion has an energy barrier of 6.9 kcal mol⁻¹, again lower than the barrier for ethylene insertion in D′·C₂H₄.

To this point we have made an account of the computational results without a critical discussion of their agreement with the experiments. According to these results, both allyl-Rh(III) species and alkyne cyclotrimerization products may be accessible with acetylene and DMAD, given the relative stabilities of the calculated intermediates and energy barriers with both alkynes. However, inspection of ligand exchange kinetics and barriers for oxidative coupling reveal that formation of B from A is slower than oxidative coupling in A to yield C (the energy barriers are 21.3 kcal mol⁻¹ for ethylene dissociation from A compared to 17.9 kcal mol⁻¹ for oxidative coupling). According to these results, the allyl-Rh(III) species 12 may form via alkyne insertion at C·C₂H₂ (Fig. 6). In the DMAD system, oxidative coupling at B′ and trapping of D′ with DMAD lead to the Rh(I) cyclotrimerization product seen experimentally. However, the calculations predict that oxidative coupling at A′ (ΔG^‡_qh = 16.3 kcal mol⁻¹) is also faster than ethylene dissociation (providing that formation of B′ follows a dissociative mechanism; dissociation energy = 19.6 kcal mol⁻¹). While we cannot offer a clear-cut explanation of the experimental observations, the calculations do show that energy barriers of the competing processes are similar enough “within the DFT error”. We propose that a delicate balance exists between these barriers, which is dependent on the alkyne and on its concentration. Thus, the detection in this work of rhodacyclopentenes 9 and 13 (Schemes 5 and 8) shows formation of intermediates of type C that are instantly trapped by water or acetonitrile molecules that compete favorably with ethylene or acetylene. In the presence of DTBAD and DMAD,^6c both metalla-cyclopentenes and -cyclopentadienes have been detected in their reactions with 1 as well as in reactions of the related TpIr(C₂H₄)₂ ^8a and Tp^Me₂Ir(C₂H₄)₂ ^6a with DMAD. In addition, formation of the rhodacycloheptene 14 (Scheme 10) in ethylene-saturated solvent supports a role for concentration, or availability of ethylene and the alkyne in the outcome of the reaction. The calculations show that 14 can be formed as the product of the insertion of ethylene into the Rh-C_sp2 bond of C·C₂H₄, which has a barrier of 14.5 kcal mol⁻¹,¹⁸ and is exergonic by 14.7 kcal mol⁻¹. Clearly, oxidative coupling at A to give C is preferred here to substitution of the second ethylene ligand to form B, which would evolve irreversibly to D. Then, excess ethylene traps C, instead of the alkyne, to form 14.

Conclusions

The reactivity of the Rh(I) complex 1 toward a range of alkynes bearing different substituents (DTBAD, acetylene, phenylacetylene, and methyl propiolate) has been systematically investigated, and the results compared with its previously reported behaviour toward DMAD. These studies reveal that both Rh(I) and Rh(III) stable products can be selectively formed depending on the nature of the alkyne.

Alkynes bearing electron-withdrawing groups at both termini of the triple bond, such as DTBAD, preferentially stabilize η⁴-diene Rh(I) species, in both open-chain and cyclic forms. In contrast, terminal alkynes (acetylene and phenylacetylene) readily evolve toward η³-allyl Rh(III) complexes. Notably, methyl propiolate, which contains only a single electron-withdrawing substituent, exhibits intermediate reactivity, leading to the formation of both η⁴-diene Rh(I) and η³-allyl Rh(III) species.

The first step in all these reactions is the substitution of one ethylene by one alkyne and the coupling of these two unsaturated ligands (the remaining ethylene and one alkyne), to readily form rhodacyclopentenes, while in the presence of excess alkyne, substitution of both ethylenes is feasible and facile oxidative coupling yields rhodacyclopentadienes, providing access to different products.

DFT calculations support these findings and reveal that oxidative coupling is faster in bis-alkyne complexes than in mixed ethylene alkyne intermediates, in both cases the oxidative coupling is not reversible, and the resulting metallacycles further stabilized by coordination of ethylene or alkyne, being the coordination of ethylene more favorable thermodynamically. However, the oxidative coupling in mixed ethylene alkyne intermediate is faster than the formation of the bis-alkyne complex. This also agrees with the different outcome of the reactions of 1 depending on the concentration of ethylene and alkyne, highlighting the favorable coupling between ethylene and alkyne, even under conditions of alkyne excess. Both alkyne concentration and solvent nature are key factors influencing the formation of these rhodacyclic species.

Experimental section

General methods

Manipulations were performed either in air or under oxygen-free dinitrogen atmosphere, by means of conventional Schlenk techniques. Solvents were freshly distilled prior to use over sodium/benzophenone or CaH₂ according to standard procedures. Microanalyses were conducted by the Microanalytical Service of the Instituto de Investigaciones Químicas (Sevilla). HRMS data were obtained on a JEOL JMS-SX 102A mass spectrometer by the Mass Spectrometry Services of the University of Seville (CITIUS). Infrared spectra were obtained by using Perkin–Elmer spectrometers, models 577 and 684. NMR characterization was carried out using Bruker DRX-500, DRX-400, AVANCE III-400R and DPX-300 spectrometers. Spectra were referenced to external SiMe₄ (δ 0 ppm) using the residual protio solvent peaks as internal standards (¹H NMR experiments) or the characteristic resonances of the solvent nuclei (¹³C NMR experiments). Spectral assignments were performed by means of routine one- and two-dimensional NMR experiments where appropriate. ¹J(C,H) coupling constants were obtained from gated-¹³C spectra. For the case of H,H coupling on a particular compound, if H_A is coupled to H_B, only the first of them appearing in the list will be accompanied by the J(AB) value. Complex TpRh(C₂H₄)₂,²² 1, was synthesized according to a procedure reported in a previous publication.^6c

Reaction of 1 with DTBAD. To a solution of 1 (200 mg, 0.54 mmol) in benzene (15 mL), DTBAD (0.366 g, 1.62 mmol) was added as solid and the mixture was stirred for 4 h at RT. Then, the solvent was evaporated off and the residual orange solid was purified by column chromatography changing the polarity of the eluent mixture hexane/diethyl ether from 10 : 1 to 1 : 1 to yield 3 (51%), 4 (4%) and 5 (12%). Excess of DTBAD could also be recovered by chromatography.

Compound 3. Yield: 0.165 g (51%). ¹H NMR (CDCl₃): δ 8.02, 7.85, 7.78, 7.45, 7.25, 6.31, 6.02 (s, d, d, s, s, s, d, 1 : 1 : 1 : 2 : 1 : 1 : 2 H, ³J_HH = 1.9 Hz, 9 CH_ar), 5.21 (t, ³J_CA,CB = 7 Hz, 1 H, H_c), 2.26 (dd, ²J_BA = 4 Hz, ³J_BC = 7 Hz, 1 H, H_b), 2.04 (m, 1 H, H_a), 1.98, 0.97 (dq, ²J_HH = 13 Hz, ³J_HH = 7 Hz, 2 H, CH₂CH₃), 1.61, 0.86 (s, 9 H each, 2 CO₂CMe₃), 1.05 (t, 3 H, CH₂CH₃). ¹³C{¹H} NMR (CDCl₃): δ 171.9, 168.3 (s, CO₂CMe₃), 143.9, 142.9, 139.6, 134.6, 134.2, 134.1, 105.1, 105.0, 104.3 (s, CH_pz), 96.7 (d, ¹J_CRh = 7 Hz, C³), 87.4 (d, ¹J_CH = 166 Hz, ¹J_CRh = 6 Hz, C²), 81.7, 78.3 (s, CO₂CMe₃), 43.1 (d, ¹J_CRh = 16 Hz, C⁴), 31.4 (d, ¹J_CH = 160, 150 Hz, ¹J_CRh = 17 Hz, C¹), 28.1, 27.5 (s, ¹J_CH = 130 Hz, 2 CO₂CMe₃), 21.0 (s, ¹J_CH = 130 Hz, CH₂CH₃), 13.1 (s, ¹J_CH = 126 Hz, CH₂CH₃). IR (nujol): v(C=O) 1704 cm⁻¹. Elemental analysis calcd (%) for C₂₅H₃₆BN₆O₄Rh: C, 50.2; H, 6.1; N, 14.1; found: C, 50.2; H, 6.0; N, 13.8.

Compound 4. Yield: 0.016 g (4%). This compound was isolated in admixture with small amounts of compound 5. ¹H NMR (CDCl₃, 25 °C): δ 8.06, 7.78, 7.70, 7.44, 6.37, 5.94 (s, 1 : 1 : 2 : 2 : 1 : 2 H, 9 CH_pz), 6.03 (s, 2 H, 2 H_a), 3.81 (s, 2 H, 2 H_b), 1.41, 0.94 (s, 18 H each, 4 CO₂CMe₃). ¹³C{¹H} NMR (CDCl₃): δ 171.5, 170.0 (s, CO₂CMe₃), 144.0, 141.7, 134.6, 134.5, 105.1, 104.7 (s, 2 : 1 : 2 : 1 : 1 : 2, CH_pz), 82.1 (d, ¹J_CH = 175 Hz, ¹J_CRh = 7 Hz, C¹), 81.2, 79.6 (s, CO₂CMe₃), 48.4 (d, ¹J_CRh = 7 Hz, C²), 46.3 (s, ¹J_CH = 138 Hz, 5 Hz, C³), 28.0, 27.6 (s, CO₂CMe₃). Elemental analysis calcd (%) for C₃₅H₅₀BN₆O₈Rh: C, 52.8; H, 6.3; N, 10.5; found: C, 53.5; H, 6.6; N, 9.5.

Compound 5. Yield: 0.051 g (12%). ¹H NMR (CDCl₃, 25 °C): δ 7.92, 7.82, 7.73, 7.53, 7.45, 7.36, 6.32, 6.10, 6.00 (s, 1 H each, 9 CH_pz), 5.50 (d, 1 H, ³J_AB = 5.5 Hz, H_a), 3.83 (d, 1 H, ³J_DC = 11 Hz, H_d), 3.41 (dd, 1 H, ³J_BC = 2.7 Hz, H_b), 3.25 (dd, H_c), 1.51, 1.47, 1.42, 1.01 (s, 9 H each, 4 CO₂CMe₃). ¹³C{¹H} NMR (CDCl₃): δ 170.2, 169.3, 166.6 (s, 2 : 1 : 1, CO₂CMe₃), 143.1, 141.7, 140.8, 134.7, 134.5, 134.2, 105.4, 105.2, 104.0 (CH_pz), 93.2 (d, ¹J_CRh = 8 Hz, C⁵), 85.4 (d, ¹J_CH = 174 Hz, ¹J_CRh = 8 Hz, C¹), 82.0, 80.9, 80.8, 79.4 (CO₂CMe₃), 46.8 (s, ¹J_CH = 130 Hz, C³), 44.1 (d, ¹J_CRh = 15 Hz, C⁶), 43.2 (s, ¹J_CH = 140 Hz, C²), 43.2 (d, ¹J_CH = 140 Hz, ¹J_CRh = 15 Hz, C⁴), 28.4, 28.1, 28.0, 27.6 (CO₂CMe₃). Elemental analysis calcd (%) for C₃₅H₅₀BN₆O₈Rh: C, 52.8; H, 6.3; N, 10.5; found: C, 52.2; H, 6.3; N, 9.9.

Compounds 6 and 7. To a solution of 1 (0.020 g, 0.054 mmol) in C₆D₆ (0.5 mL) in a NMR tube at 8 °C, DTBAD was added (0.012 g, 1 equiv.) and the reaction monitored by NMR to show initial formation of compound 6. If an excess of DTBAD (0.030 g, 2.5 equiv.) was added, the ¹H NMR recorded upon mixture revealed instead a mixture of 6 and 7 in 2 : 1 ratio, before full evolution of the mixtures to the final compounds 3 and 8 (25 °C, 4 h). No elemental analysis or HRMS data could be obtained.

Selected NMR data for compound 6

¹H NMR (400 MHz, C₆D₆): δ 3.97 (AA'XX’ spin system, 4 H, CH₂=CH₂), 3.07, 3.02, 2.72, 1.92 (m, m, m, m, 1 H each, Rh-CH₂CH₂–). ¹³C{¹H} NMR (101 MHz, C₆D₆): δ 170.9 (RhC_q), 135.3 (C_qCH₂), 79.5 (d, CH₂=CH₂), 36.6 (s, RhCH₂CH₂), 21.9 (d, J_RhC = 20 Hz, RhCH₂CH₂).

Selected NMR data for compound 7

¹H NMR (400 MHz, C₆D₆): δ 4.32 (s, 4 H, CH₂=CH₂).

Compound 8. This compound was synthesized according to the procedure described above for the reaction of 1 (0.015 g, 0.04 mmol) with DTBAD (0.027 g, 0.12 mmol) but using THF (4 h at RT) instead of benzene as the solvent. Compound 8 was purified by column chromatography on silica gel using hexane/diethyl ether (10 : 1) as eluent. Yield: 0.008 g (24%). Slow evaporation of a solution of the crude mixture in hexane : dichloromethane provided crystals suitable for X-ray. ¹H NMR (300 MHz, CDCl₃): δ 7.97, 7.93, 7.76, 7.44, 6.33, 5.95 (d, d, d, d, t, t, 1 : 2 : 1 : 2 : 1 : 2 H, ³J_HH = 2.1 Hz, 9 CH_pz), 2.46, 1.01 (AA'XX’ spin system, 2 H each, ²J_HH = 10.3 Hz, 2 CH₂), 1.57, 0.92 (s, 18 H each, 4 CO₂CMe₃). ¹³C{¹H} NMR (75 MHz, CDCl₃): δ 170.8, 164.8 (s, CO₂CMe₃), 144.6, 142.0, 134.2, 134.1, 104.8, 104.1 (s, 2 : 1 : 2 : 1 : 1 : 2, CH_pz), 92.9 (d, ¹J_CRh = 7 Hz, C_q-CO₂CMe₃), 82.5, 79.4 (s, CO₂CMe₃), 46.5 (d, ¹J_CRh = 15 Hz, C_q-CO₂CMe₃), 28.1, 27.4 (s, CO₂CMe₃), 23.4 (s, CH₂). Elemental analysis calcd (%) for C₃₅H₅₀BN₆O₈Rh: C, 52.8; H, 6.3; N, 10.5; found: C, 52.5; H, 6.1; N, 10.5. HRMS (FAB): m/z calcd for C₃₅H₅₀BN₆O₈NaRh: 819.2736; found: 819.2756 [M + Na]⁺.

Compound 9. This compound was obtained by reaction between 1 (0.010 g, 0.03 mmol) and 1 eq. of DTBAD (0.006 g, 0.03 mmol) in THF/H₂O (1 mL, 9/1) for 4 h at RT. Flash chromatography using hexane : Et₂O (1 : 1) yielded compound (0.009 g, 92%). Crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of compound 9 in chloroform. ¹H NMR (400 MHz, CDCl₃): δ 7.80, 7.66, 7.64, 7.63, 7.57, 7.31 6.19, 6.17, 6.04 (d, d, d, d, d, d, t, t, t, 1 H each, ³J_HH = 2.3 Hz, 9 CH_pz), 4.26 (brs, 2 H, H₂O), 2.98, 2.88 (m, 2 H each, RhCH₂CH₂), 1.38, 0.89 (s, s, 9 H each, CO₂CMe₃). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 177.9, 163.4 (CO₂CMe₃), 173.3 (d, ¹J_CRh = 31 Hz, RhC_q), 143.6, 141.6, 140.4, 135.3, 134.6, 134.4, 105.3, 105.1, 104.9 (d, s, s, s, s, s, s, d, s, J_CRh = 2 Hz, CH_pz), 140.4 (C_q), 79.9, 79.8 (CO₂CMe₃), 36.8 (RhCH₂CH₂), 28.4, 27.9 (CO₂CMe₃), 21.6 (d, ¹J_CRh = 23 Hz, RhCH₂CH₂). Elemental analysis calcd (%) for C₂₃H₃₄BN₆O₈Rh: C, 46.9; H, 5.8; N, 14.3; found: C, 46.9; H, 6.0; N, 14.4.

Reaction of 3 with excess of DMAD. Compound 3 (0.050 g, 0.08 mmol) was dissolved in benzene (3 mL) and excess DMAD (0.06 mL, 0.50 mmol) was added. The mixture was heated for 12 h at 65 °C, the solvent was evaporated, and the residue analyzed by ¹H NMR to contain a mixture of compounds 10,^6c 11, 12 and 13, which were separated by column chromatography on silica gel using hexane/diethyl ether as eluent.

Compound 10. Yield: 0.009 g (38%). ¹H NMR (300 MHz, CDCl₃): δ 6.58 (dd, 1 H, ³J_HH = 17.8, 11.1 Hz, H_a), 5.52 (d, 1 H, ³J_HH = 17.8 Hz, H_c), 5.44 (d, 1 H, ³J_HH = 10.9 Hz, H_b), 2.39 (q, 2 H, ³J_HH = 7.5 Hz, CH₂CH₃), 1.54, 1.50 (s, s, 9 H each, CO₂CMe₃), 1.06 (t, 3 H, ³J_HH = 7.5 Hz, CH₂CH₃). ¹³C{¹H} NMR (75 MHz, CDCl₃): δ 167.5, 166.7 (CO₂CMe₃), 139.0 (C_q-CH₂), 135.4 (C_q-CH), 130.0 (CH=CH₂), 121.4 (CH=CH₂), 81.9, 81.3 (CO₂CMe₃), 28.2, 28.1 (CO₂CMe₃), 21.8 (CH₂CH₃), 13.4 (CH₂CH₃). HRMS (FAB): m/z calcd for C₁₆H₂₆O₄Na: 305.1729; found: 305.1725 [M + Na]⁺.

Compound 11. Yield: 0.018 g (25%). ¹H NMR (400 MHz, CDCl₃): δ 7.91, 7.80, 7.77, 7.75, 7.53, 7.37, 6.37, 6.06, 6.02 (d, d, d, d, d, d, t, t, t, 1 H each, ³J_HH = 2.3 Hz, 9 CH_pz), 7.07, 3.46 (d, d, 1 H each, ³J_HH = 11.6 Hz, CH_allyl), 4.12 (d, 1 H, ²J_HRh = 2.8 Hz, Rh-CH), 3.77, 3.76, 3.53, 2.75 (s, 3 H each, CO₂Me), 2.50 (m, 2 H, CH₂CH₃), 1.51, 0.74 (s, 9 H each, CO₂CMe₃), 1.15 (t, 3 H, ³J_HH = 7.5 Hz, CH₂CH₃). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ (ppm) = 176.5, 171.6, 167.5, 167.0, 165.1, 164.7 (CO₂Me + CO₂CMe₃), 148.5 (C_q), 145.2, 144.0, 143.4, 136.1, 135.3, 134.8, 106.0, 105.3, 104.9 (CH_pz), 141.3 (C_q), 136.7 (C_q), 136.6 (C_q), 101.7, 67.3 (d, d, ¹J_CRh = 5, 8 Hz resp., CH_allyl), 81.6 (d, ¹J_CRh = 9 Hz, C_q–allyl), 52.5, 52.4, 51.8, 50.9 (CO₂Me), 36.5, 36.3 (CO₂CMe₃), 28.3, 26.8 (CO₂CMe₃), 23.7 (CH₂CH₃), 13.2 (CH₂CH₃). Elemental analysis calcd (%) for C₃₇H₄₈BN₆O₁₂Rh: C, 50.3; H, 5.5; N, 9.5; found: C, 50.0; H, 5.4; N, 9.1. HRMS (FAB): m/z calcd for C₃₇H₄₈BN₆O₁₂Rh: 882.2478; found: 882.2495 [M]⁺.

Compound 12. To a solution of 1 (0.100 mg, 0.27 mmol) in benzene (5 ml) in a Schlenk tube, acetylene was bubbled into the solution through a needle for approximately 1 min and the tube was sealed and left under stirring for 2 h at RT. After evaporation of solvent, compound 12 was obtained as pale-yellow solid after purification through silica gel with hexane. Yield: 93 mg (87%). ¹H NMR (400 MHz, CDCl₃): δ 7.87, 7.68, 7.62, 7.46, 6.24, 6.23, 6.13 (d, m, d, d, t, t, t, 1 : 3 : 1 : 1 : 1 : 1 : 1 H, ³J_HH = 2.1 Hz, 9 CH_pz), 5.37 (m, 1 H, H_e), 5.31 (m, 1 H, H_f), 5.12 (m, 1 H, H_c), 4.97 (dt, 1 H, J_HH = 7.2, 2.1 Hz, H_d), 3.34 (ddt, 1 H, J_HH = 7.4, 1.3, 0.7 Hz, H_b), 2.57 (m, 1 H, H_g), 2.35 (dtd, 1 H, J_HH = 14.5, 1.9, 1.0 Hz, H_h), 1.76 (ddt, 1 H, J_HH = 14.5, 1.9, 1.0 Hz, H_a). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 143.0, 141.3, 139.1, 135.3, 134.4, 134.1, 105.5, 105.2, 104.6 (d, d, s, s, s, s, s, s, s, ¹J_CRh = 2 Hz, CH_pz), 139.6 (C⁵), 127.5 (C⁴), 103.1 (d, ¹J_CRh = 6 Hz, C²), 73.2 (d, ¹J_CRh = 12 Hz, C³), 40.8 (d, ¹J_CRh = 12 Hz, C¹), 30.6 (d, ¹J_CRh = 21 Hz, C⁶). Elemental analysis calcd (%) for C₁₅H₁₈BN₆Rh: C, 45.5; H, 4.6; N, 21.2; found: C, 46.0; H, 4.7; N, 20.8. HRMS (FAB): m/z calcd for C₁₅H₁₈BN₆Rh: 396.0741; found: 396.0746 [M]⁺.

Compound 13. This compound was synthesized by bubbling acetylene to a solution of 1 (0.030 g, 0.08 mmol) in acetonitrile (2 mL) in a Schlenk tube for 1 minute and letting the mixture stir for 1 h at RT. Then the solvent was evaporated and the crude solid was washed with cold hexane. Yield: 0.020 g (62%). ¹H NMR (300 MHz, CDCl₃): δ 7.72, 7.69, 7.68, 7.60, 7.53, 7.45, 6.22, 6.20, 6.04 (d, d, d, d, d, d, t, t, t, 1 H each, ³J_HH = 2.3 Hz, 9 CH_pz), 6.82 (m, 1 H, RhCH), 6.24 (m, 1 H, RhCH=CH), 2.71, 2.55, 2.40 (m, m, s, 1 : 2 : 1 H, RhCH₂CH₂), 2.23 (s, 3 H, MeCN). ¹³C{¹H} NMR (75 MHz, CDCl₃): δ 150.4 (d, ¹J_CRh = 30 Hz, RhCH), 141.0, 140.4, 140.0, 135.2, 134.7, 134.6, 105.1, 105.0 (1 : 1 : 1 : 1 : 1 : 1 : 2 : 1, CH_pz), 140.3 (RhCH=CH), 36.7 (RhCH₂CH₂), 104.9 (MeCN), 22.8 (d, ¹J_CRh = 23 Hz, RhCH₂CH₂), 4.2 (MeCN). Elemental analysis calcd (%) for C₁₅H₁₉BN₇Rh: C, 43.8; H, 4.7; N, 23.9; found: C, 43.7; H, 4.7; N, 24.0. HRMS (FAB): m/z calcd for C₁₅H₁₈BN₇Rh: 410.0766; found: 410.0760 [M + H]⁺.

Compound 14. To a solution of 1 (0.010 g, 0.027 mmol) in CH₂Cl₂ (15 mL) in a Schlenk tube at −60 °C, ethylene was bubbled through for 2.5 minutes, after which time a tiny bubbling of acetylene was carried out for 2.5 minutes, without stopping the stream of ethylene. Then, the tube was sealed and left under stirring for 30 minutes at low temperature and then for 30 minutes at RT. After this period, the solvent was removed under reduced pressure and NMR monitoring of the crude product revealed the formation of 14 and 12 in 90 : 10 ratio. Compound 14 was isolated by column chromatography on silica gel using pentane as eluent although it could not be obtained in a completely pure form. ¹H NMR (400 MHz, CDCl₃): δ 7.91, 7.67, 7.54, 7.08, 6.25, 6.05 (s, d, d, brs, t, t, 2 : 2 : 1 : 1 : 2 : 1 H, ³J_HH = 2.0 Hz, 9 CH_pz), 3.94 (m, 2 H, CH), 3.33, 3.26 (m, m, 2 H each, RhCH₂CH₂), 1.39, 0.25 (m, m, 2 H each, RhCH₂CH₂). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 140.5, 138.3, 134.5, 134.3, 104.9, 104.7 (s, s, s, s, d, s, J_CRh = 3.5 Hz, CH_pz), 69.7 (d, ¹J_CH = 159 Hz, J_CRh = 5 Hz, CH), 25.9 (d, ¹J_CH = 130.2 Hz, ¹J_CRh = 4 Hz, RhCH₂CH₂), −17.4 (d, ¹J_CH = 141 Hz, ¹J_CRh = 16 Hz, RhCH₂CH₂). HRMS (FAB): m/z calcd for C₁₅H₂₁BN₆Rh: 399.0970; found: 399.0963 [M + H]⁺.

Compound 15. To a solution of 13 (0.040 g, 0.10 mmol) in CH₂Cl₂ (10 mL) in a Schlenk tube at −20 °C, ethylene was bubbled through a needle for 3 minutes and the tube was sealed and left under stirring for 1 h at RT and then for 5 h at 60 °C. After evaporation of solvent, compound 15 was obtained as pale-yellow solid after purification through silica gel using pentane as eluent. Yield: 0.022 g (57%). ¹H NMR (400 MHz, C₆D₆): δ 7.93, 7.54, 7.43, 7.03, 6.02, 5.84 (brs, brs, brs, brs, brs, brs, 1 : 1 : 2 : 2 : 1 : 2 H, 9 CH_ar), 5.02 (q, ³J_HH = 7 Hz, H_c), 4.61 (t, ³J_DC,DE = 7 Hz, H_d), 2.55 (m, 1 H, H_e), 2.01 (dd, ²J_BA = 3.2 Hz, ³J_BC = 7 Hz, H_b), 1.39 (m, 1 H, H_a), 1.45, 1.00 (m, m, 1 H each, CH₂CH₃), 1.00 (m, 3 H, CH₂CH₃). ¹³C{¹H} NMR (101 MHz, C₆D₆): δ 142.9, 139.6, 134.3, 105.2 (1 : 2 : 3 : 3, CH_pz), 89.5 (d, ¹J_CRh = 5.9 Hz, C²), 83.8 (d, ¹J_CRh = 6 Hz, C³), 46.0 (d, ¹J_CRh = 17.2 Hz, C⁴), 29.6 (d, ¹J_CRh = 16.9 Hz, C¹), 22.1 (CH₂CH₃), 17.1 (CH₂CH₃). Elemental analysis calcd (%) for C₁₅H₂₀BN₆Rh: C, 45.3; H, 5.1; N, 21.1; found: C, 45.4; H, 5.3; N, 20.8. HRMS (FAB): m/z calcd for C₁₅H₂₁BN₆Rh: 399.0976; found: 399.0970 [M + H]⁺.

Compound 16. This compound was obtained by reacting compound 1 (0.030 g, 0.08 mmol) with phenylacetylene (0.025 mL, 0.24 mmol) in 3 mL of benzene. The solution was left under stirring overnight at RT, then the solvent was evaporated under vacuum and the residue purified through silica gel (pentane) to give a yellow solid. Yield: 0.019 g (44%). ¹H NMR (400 MHz, CDCl₃): δ 7.91, 7.70, 7.69, 7.62, 7.44, 7.14, 6.26, 6.17, 6.09 (d, 1 H each, ³J_HH = 2 Hz, 9 CH_pz), 7.62 (m, 5 H, 5 CH_ar), 7.34, 7.29, 7.14 (m, 2 : 1 : 2, 5 CH_ar), 6.13 (s, 1 H, H_d), 5.83 (dd, 1 H, ²J_HH = 11.0, 7.5 Hz, H_c), 3.39 (d, 1 H, ³J_HH = 7.5 Hz, H_b), 3.18, 2.80 (d, 1 H each, ³J_HH = 13.6 Hz, RhCH₂), 1.92 (d, 1 H, J_HH = 11.0 Hz, H_a). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 147.7, 143.9 (C_qPh), 143.2, 140.6, 139.3, 135.3, 134.5, 134.1, 105.4, 105.2, 104.7 (CH_pz), 139.4 (C⁵), 128.2, 128.1, 126.9, 126.0, 125.8, 125.7 (2 : 2 : 1 : 2 : 2 : 1, CH_Ph), 125.3 (C⁴), 100.1 (d, ¹J_CRh = 7 Hz, C²), 83.22 (d, ¹J_CRh = 11 Hz, C³), 39.4 (d, ¹J_CRh = 13 Hz, C¹), 29.4 (d, ¹J_CRh = 21 Hz, C⁶). Elemental analysis calcd (%) for C₂₇H₂₆BN₆Rh: C, 59.2; H, 4.8; N, 15.3; found: C, 59.5; H, 4.9; N, 15.0. HRMS (FAB): m/z calcd for C₂₇H₂₆BN₆Rh: 548.1367; found: 548.1387 [M]⁺.

Reaction of 1 with methyl propiolate. To a solution of 1 (0.200 g, 0.54 mmol) in benzene (10 mL), neat MP (0.140 mL, 1.60 mmol) was added dropwise and the mixture was stirred for 4 h at RT. NMR of the crude product revealed a complex mixture of derivatives from which, after removal of volatiles, column chromatography (gradually changing the polarity of the pentane/diethyl ether eluent mixture from 8 : 1 to 2 : 1) yielded 17 (9%), 18 (4%) and 19 (48%).

Compound 17. Yield: 0.023 g (9%). ¹H NMR (400 MHz, CDCl₃): δ 8.07, 7.78, 7.70, 7.56, 7.54, 7.25, 6.34, 6.06 (s, 1 : 1 : 1 : 1 : 1 : 1 : 1 : 2 H, 9 CH_pz), 6.01 (d, ³J_HH = 5.1 Hz, H_d), 5.23 (td, ³J_HH = 7.3, 5.1 Hz, H_c), 3.28 (s, 1 CO₂Me), 2.17, 1.83 (m, m, 1 H each, H_a and H_b), 2.12, 1.21 (m, m, 1 H each, CH₂CH₃), 0.89 (t, 3 H, ³J_HH = 7.5 Hz, CH₂CH₃). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 177.4 (s, CO₂Me), 142.5, 141.8, 139.7, 134.8, 134.4, 105.1, 104.3 (s, 1 : 1 : 1 : 1 : 2 : 2 : 1, CH_pz), 86.4 (d, ¹J_CRh = 5 Hz, C³), 86.3 (d, ¹J_CRh = 6 Hz, C²), 51.3 (s, CO₂Me), 47.7 (d, ¹J_CRh = 17 Hz, C⁴), 31.9 (d, ¹J_CRh = 17 Hz, C¹), 22.2 (CH₂CH₃), 15.0 (CH₂CH₃). Elemental analysis calcd (%) for C₁₇H₂₂BN₆O₂Rh: C, 44.8; H, 4.9; N, 18.4; found: C, 44.3; H, 4.8; N, 18.9. HRMS (FAB): m/z calcd for C₁₇H₂₂BN₆O₂NaRh: 479.0850; found: 479.0851 [M + Na]⁺.

Compound 18. Yield: 0.012 g (4%). ¹H NMR (400 MHz, CDCl₃): δ 8.06, 7.79, 7.68, 7.51, 6.39, 6.02 (d, d, d, d, t, t, 1 : 1 : 2 : 2 : 1 : 2 H, ³J_HH = 2.3 Hz, 9 CH_pz), 6.16 (s, 2 H, CH–CH), 3.41 (s, 6 H, 2 CO₂Me), 2.36, 0.96 (m, m, 2 H each, CH₂CH₂). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 175.1 (CO₂Me), 143.0, 142.1, 134.8, 105.5, 104.8 (2 : 1 : 3 : 2 : 1, CH_pz), 83.6 (d, ¹J_CRh = 7 Hz, CH–CH), 51.6 (CO₂Me), 47.7 (d, ¹J_CRh = 15 Hz, CCO₂Me), 23.1 (CH₂). Elemental analysis calcd (%) for C₁₉H₂₂BN₆O₄Rh: C, 44.6; H, 4.3; N, 16.4; found: C, 44.9; H, 4.5; N, 16.2. HRMS (FAB): m/z calcd for C₁₉H₂₂BN₆O₄NaRh: 535.0748; found: 535.0750 [M + Na]⁺.

Compound 19. Yield: 0.132 g (48%). ¹H NMR (400 MHz, CDCl₃): δ 7.84, 7.74, 7.66, 7.61, 7.55, 6.24, 6.19, 6.13 (s, 1 : 1 : 1 : 1 : 2 : 1 : 1 : 1 H, 9 CH_pz), 6.60 (s, 1 H, H_d), 6.20 (dd, 1 H, ³J_HH = 12.1, 7.8 Hz, H_c), 3.71, 3.53 (s, 3 H each, 2 CO₂Me), 3.60, 2.14 (d, d, 1 H each, ³J_HH = 7.8 Hz, ³J_HH = 12.1 Hz, H_a and H_b), 2.72, 2.63 (d, 1 H each, ²J_HH = 14.5 Hz, RhCH₂). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 172.9, 166.1 (s, CO₂Me), 143.7, 141.2, 140.3, 135.1, 134.8, 134.3, 105.9, 105.7, 104.6 (s, CH_pz), 141.8 (C⁵), 137.5 (C⁴), 106.7 (d, ¹J_CRh = 6 Hz, C²), 67.9 (d, ¹J_CRh = 12 Hz, C³), 51.9, 51.6 (s, CO₂Me), 47.1 (d, ¹J_CRh = 10 Hz, C¹), 27.9 (d, ¹J_CRh = 20 Hz, Rh-CH₂). Elemental analysis calcd (%) for C₁₉H₂₂BN₆O₄Rh: C, 44.6; H, 4.3; N, 16.4; found: C, 44.8; H, 4.2; N, 16.1. HRMS (FAB): m/z calcd for C₁₉H₂₂BN₆O₄NaRh: 535.0748; found: 535.0740 [M + Na]⁺.

Computational details

Calculations were performed at the DFT level with the Gaussian09 program.²³ The Head-Gordon hybrid functional ωB97X-D,²⁴ which includes empirical dispersion, was used throughout the computational study. Geometry optimizations were carried out in the gas phase, without geometry constraints using the 6-31G(d,p) basis set²⁵ to represent the C, H, N and O atoms and the Stuttgart/Dresden Effective Core Potential and its associated basis set (SDD)²⁶ to model the Rh atoms (BS1). The stationary points of the Potential Energy Surface and their nature as minima or saddle points (TS) were characterized by vibrational analysis, which also gave gas-phase enthalpies (H), entropies (S) and Gibbs energies (G). The minima connected by a given transition state were determined by Intrinsic Reaction Coordinate (IRC) calculations or by perturbing the transition states along the TS coordinate and optimizing to the nearest minimum. The energies reported in the main text were obtained from single point calculations on the geometries previously optimized at the BS1 level using the Dunning's triple-ζ basis set cc-pVTZ ²⁷ for C, H, N and O and also including solvent (dichloromethane) corrections with the SMD continuum model²⁸ (BS2). Free energy values discussed herein include corrections of the effect of the rigid-rotor harmonic oscillator treatment on thermodynamic magnitudes: ΔG_qh. Quasi-harmonic corrections have been carried out with the GoodVibes²⁹ software using the approximations of Grimme³⁰ for entropy and of Head-Gordon for enthalpy.³¹

Author contributions

G. B., J. E. C and V. S.: data curation, formal analysis, and investigation; J. J. M. and J. L.-S.: formal analysis (DFT); J. C. and K. M.: formal analysis (X-Ray); N. R.: investigation and writing – review & editing; J. L.-S., M. P. and L. L. S.: conceptualization, investigation, methodology, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the manuscript, in the “Experimental” section, and in the supplementary information (SI). Supplementary information: X-ray structures for 3, 4, 5, 8 and 9, computational details and NMR spectra for compounds 3, 4, 5, 8, 9, 10, 11, 13, 14, 15, 16, 17, 18 and 19 (PDF). The supplemental file “xyz Coordinates of calculated species” contains the computed Cartesian coordinates of all of the molecules reported in this study. The file may be opened as a text file to read the coordinates, or opened directly by a molecular modeling program such as Mercury (version 3.3 or later, https://www.ccdc.cam.ac.uk/pages/Home.aspx) for visualization and analysis (doc). See DOI: https://doi.org/10.1039/d6dt00618c.

CCDC 1411328–1411332 contain the supplementary crystallographic data for this paper.^32a–e

Acknowledgements

Financial support from MCIN/AEI/10.13039/501100011033/FEDER, UE (PID2019-104159GB-I00 and PID2022-136570OB-I00) is gratefully acknowledged. The use of computational facilities at the Supercomputing Centre of Galicia (CESGA) and the Centro de Servicios de Informática y Redes de Comunicaciones (CSIRC, UGRGRID), Universidad de Granada (Spain) is gratefully acknowledged.

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